Reference pad for position sensing

ABSTRACT

A method for position tracking includes attaching a location pad to a body of a subject and introducing into the body of the subject a first position transducer. A procedure is performed on the body of the subject using a medical tool, to which a second position transducer is fixed. Magnetic fields are transmitted between the location pad and first and second position transducers. Responsively to the magnetic fields, first and second position signals are generated. The signals are indicative of coordinates of the first and second position transducers relative to the location pad. The first and second position signals are processed so as to determine a disposition of the tool relative to the first position transducer.

FIELD OF THE INVENTION

The present invention relates generally to tracking the spatial coordinates of objects used during medical procedures, and specifically to methods and devices for tracking the position and orientation of medical tools and intrabody devices.

BACKGROUND OF THE INVENTION

Various methods and systems are known in the art for tracking the coordinates of objects involved in medical procedures.

For example, U.S. Pat. Nos. 5,391,199 and 5,443,489 to Ben-Haim, whose disclosures are incorporated herein by reference, describe systems wherein the coordinates of an intrabody probe are determined using one or more field transducers, such as a Hall effect device, coils, or other antennae carried on the probe. Such systems are used for generating location information regarding a medical probe or catheter. A sensor, such as a coil, is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations.

PCT Patent Publication WO 96/05768, U.S. Pat. No. 6,690,963, and U.S. patent application Ser. No. 09/414,875, all to Ben-Haim et al. (published as U.S. Patent Application Publication US 2002/0065455), whose disclosures are incorporated herein by reference, describe a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates.

U.S. Pat. No. 6,239,724 to Doron et al., whose disclosure is incorporated herein by reference, describes a wireless, telemetry system for providing coordinates of an intrabody object. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.

The above-mentioned U.S. patent application Ser. No. 10/029,473 to Govari, published as U.S. Patent Application Publication 2003/0120150, describes a system wherein a wireless transponder is fixed to an object. The transponder includes at least one sensor coil, in which a signal current flows responsive to the electromagnetic fields, and a power coil, which receives the RF driving field and conveys electrical energy from the driving field to power the transponder. The power coil also transmits an output signal responsive to the signal current to a signal receiver, which processes the signal to determine coordinates of the object. In an embodiment of the invention, the object is a hip joint.

U.S. Pat. No. 6,618,612 to Acker et al., whose disclosure is incorporated herein by reference, describes a system wherein reference field transducers are independently movable with respect to one another to desired positions close to or mounted upon the body. Calibration transducers determine the relative positions of the field transducers with respect to one another after they are located in their desired positions. From the detected fields, the relative disposition of the probe with respect to the reference field transducers is determined.

U.S. Pat. No. 6,332,089 to Acker et al., whose disclosure is incorporated herein by reference, describes a system wherein a site probe is placed within the body of a patient and an instrument probe is guided within the body. One or more fields are transmitted to or from each of the probes, which are adapted to detect each such field. The relative disposition of the site probe and the instrument probe is determined from the properties of the detected fields. Furthermore, the instrument probe is directed toward the site probe on the basis of the so determined relative disposition. As described by Acker, relative disposition may mean relative position and/or orientation.

U.S. Patent Application Publication 2004/0068178 to Govari, whose disclosure is incorporated herein by reference, describes a system wherein primary radiators are driven by a control unit to track the positions of a plurality of secondary radiators with respect to the primary radiators. The secondary radiators are optionally movable, and are driven to track the position of the probe with respect to the secondary radiators. A calculation is then performed to determine the corresponding position of the probe with respect to the fixed locations of the primary radiators.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide magnetic tracking systems for use in tracking the positions of objects related to a medical procedure, such as medical tools and intrabody devices. The system comprises one or more location pads attached to the body and one or more position transducers that are inserted into the body. In some embodiments, the location pads transmit magnetic fields, which are received by the transducers. In other embodiments, the transducers inside the body transmit magnetic fields, which are received by the location pads. In both cases, the received field amplitudes are used in determining the coordinates of the transducers in the body relative to one or more of the location pads.

Typically, each location pad is attached to the body surface close to the area in which the position transducer is located. As a result, accurate coordinates may be determined while transmitting relatively weak magnetic fields, and interference of metal objects with the tracking system is reduced. There is no limitation on movement of the patient's body during the medical procedure, since the location pad moves together with the body.

In some embodiments of the present invention, one of the position transducers is fixed to a structure inside the body, and another position transducer is attached to a surgical tool. Both the fixed transducer and the tool transducer transmit or receive magnetic fields to or from the same location pad. By processing the received field amplitudes, the coordinates of the tool and the fixed transducer relative to the location pad are determined, and thus the coordinates of the tool relative to the fixed transducer is known. The relative coordinates may be used to guide a medical practitioner in manipulating the tool to perform a medical procedure on the body structure to which the position transducer inside the body is fixed.

In some embodiments, these systems are used in orthopedic procedures, such as implantation of implants such as screws, nails, rods or prosthetic joints. For this purpose, wireless or wired magnetic position transducers may be inserted into the patient's bone, into prosthetic implants and into tools used during surgery. The tracking system determines the coordinates of the transducers, and thus enables the surgeon to visualize the locations and orientations of these elements while reducing or eliminating the need for intraoperative X-ray imaging. Implanted position transducers may also be used in post-operative follow-up. In other embodiments, body-surface location pads are used in conjunction with position transducers in body structures and devices used in other medical procedures, such as endoscopy and cardiovascular catheterization.

In embodiments of the present invention, the origin of the coordinate system in which the transducer coordinates are found is “floating,” i.e., it moves with the patient without necessarily having a fixed frame of reference in space. In some embodiments, when multiple location pads are used, one of the pads may be chosen as the primary, floating origin for the coordinate system. The other pads may then contain position transducers, for use in determining the coordinates of these pads relative to the primary origin. The positions of all pads and transducers may thus be monitored within a single coordinate system, based on the primary floating origin.

Additionally or alternatively, the coordinates of the location pads (and hence of the position transducers) may be determined in a fixed, external frame of reference, such as an operating table, by transmitting magnetic fields between the location pads and a field generator or receiver in a known location in the external frame of reference.

In some embodiments, implants, tools, and body surface pads convey position information by wireless means to a wireless control unit. The wireless unit is coupled to a computer that processes the position signals and determines the relative dispositions. Power to operate the transducers and pad coils may be provided by batteries or by wireless inductive signals.

In further embodiments, to the location pads may take the form of one or more rings or tapes, which are attached to the body near the surgical site. Each ring or tape comprises multiple coils spaced along its length. The coordinates of target objects are determined by sensing the fields that are generated or received by the coil that gives the strongest signal—typically whichever coil is closest to the object in question.

There is therefore provided, in accordance with an embodiment of the present invention, a tracking system, including:.

a location pad, which is configured to be attached to a body of a subject and to generate a magnetic field within the body;

a first position transducer, which is adapted to be introduced into the body of the subject and responsively to the magnetic field, to generate and transmit a first position signal that is indicative of first coordinates of the first position transducer relative to the location pad;

a second position transducer, which is fixed to a medical tool adapted for performing a procedure on the body of the subject and, responsively to the magnetic field, to generate and transmit a second position signal that is indicative of second coordinates of the second position transducer relative to the location pad; and

a system controller, which is coupled to receive and process the first and second position signals so as to determine a disposition of the tool relative to the first position transducer.

In some embodiments, the location pad is one of at least first and second location pads, which are configured to be attached to the body at respective locations and to generate respective magnetic fields with respective ranges, and the first and second position transducers are adapted respectively to generate the first and second position signals responsively to the magnetic field within whose range they are located. In some embodiments, the system controller is adapted to determine pad coordinates of the first location pad relative to the second location pad, and thereby to register the first and second coordinates in a common reference frame. In further embodiments, the first location pad is adapted to generate a third position signal responsively to the magnetic field of the second location pad, and the system controller is coupled to receive and process the third position signal in order to determine the pad coordinates. In still further embodiments, the system includes a third location pad, and the second location pad is adapted to generate a fourth position signal responsively to a magnetic field of the third location pad, and the system controller is coupled to receive and process the fourth position signal in order to register the first, second and third location pads in the common reference frame.

In some embodiments, the first and second location pads are mounted on a single unit, which is configured to be attached to the body of the subject.

In additional embodiments, a third position transducer is fixed in a frame of reference external to the body of the subject and is adapted to generate a third position signal responsively to the magnetic field, and the system controller is coupled to receive and process the third position signal so as to determine the first and second coordinates in the external frame of the reference.

Typically, the location pad is adapted to be affixed to a surface of the body of the subject.

In disclosed embodiments, the location pad includes a plurality of concentric, orthogonal magnetic field generating coils.

In some embodiments, at least one of the first and second position transducers includes one or more transducer coils, which are adapted to sense the magnetic fields so as to generate at least one of the first and second position signals.

In a disclosed embodiment, a driving antenna is adapted to radiate a radio frequency (RF) electromagnetic field, and the location pad includes a power coil, which is coupled to receive the RF electromagnetic field and thereby to provide power for generating the magnetic field. Alternatively, the location pad includes an internal power source to provide power for generating the magnetic field.

Typically, the first and second position transducers include wireless transmitters for communicating with the system controller.

There is also provided, in accordance with an embodiment of the present invention, a tracking system, including:

a first position transducer, which is adapted to be introduced into a body of a subject and to generate a first magnetic field;

a second position transducer, which is fixed to a medical tool adapted for performing a procedure on the body of the subject and which is adapted to generate a second magnetic field;

a location pad, which is configured to be attached to the body of the subject, to receive the first and second magnetic fields, and to generate and transmit first and second position signals that are indicative of respective first and second coordinates of the first and second position transducers relative to the location pad; and

a system controller, which is coupled to receive and process the first and second position signals so as to determine a disposition of the tool relative to the first position transducer.

Typically, the location pad is adapted to transmit the first and second position signals over a wireless connection to the system controller.

There is also provided, in accordance with an embodiment of the present invention, a method for position tracking, including:

attaching a location pad to a body of a subject;

introducing into the body of the subject a first position transducer;

performing a procedure on the body of the subject using a medical tool, to which a second position transducer is fixed;

transmitting magnetic fields between the location pad and first and second position transducers;

responsively to the magnetic fields, generating first and second position signals that are indicative of coordinates of the first and second position transducers relative to the location pad; and

processing the first and second position signals so as to determine a disposition of the tool relative to the first position transducer.

Typically, transmitting the magnetic fields includes transmitting the magnetic fields from the location pad, and generating the first and second position signals includes generating the first and second position signals responsively to the magnetic fields received at the first and second position transducers.

Alternatively, transmitting the magnetic fields includes transmitting the magnetic fields from the first and second position transducers, and generating the first and second position signals includes generating the first and second position signals responsively to the magnetic fields received at the location pad from the first and second position transducers.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic, pictorial illustration of a magnetic tracking system used in surgery, in accordance with an embodiment of the present invention;

FIG. 1B is a schematic, pictorial illustration of a magnetic tracking system used in surgery, in accordance with an alternative embodiment of the present invention;

FIG. 1C is a schematic, pictorial illustration of a magnetic tracking system used in surgery, in accordance with another alternative embodiment of the present invention;

FIGS. 2A and 2B are schematic, partly sectional illustrations, showing insertion of implantable position transducers into the bone of a patient, in accordance with an embodiment of the present invention;

FIGS. 3A and 3B are schematic, pictorial illustrations showing details of position transducers, in accordance with embodiments of the present invention;

FIG. 4 is a schematic, pictorial illustration showing details of a body surface pad, in accordance with an embodiment of the present invention;

FIG. 5 is a schematic, pictorial illustration showing a surgical tool and a position transducer used to track coordinates of the tool, in accordance with an embodiment of the present invention; and

FIG. 6 is a schematic, pictorial illustration showing an alternate configuration of body surface pads, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1A is a schematic, pictorial illustration of a magnetic tracking system 20 for use in surgery, in accordance with an embodiment of the present invention. In the pictured embodiment, a surgeon 22 is performing a procedure that involves maneuvering a tool 24 to positions in contact with, or relative to, implantable devices or probes 26 and 28, hereinafter referred to as implants 26 and 28. In the example of FIG. 1A, implants 26 and 28 have been introduced into the body at a surgical site, which is located in a leg 30 of a patient 32. In this example, implants 26 and 28 have been introduced into the patient's tibia and femur near the knee, for use in guiding the surgeon in performing a procedure on the knee joint using tool 24. Exemplary methods and tools for use in inserting the implants into the bone are described in U.S. patent application No. (Applicant's Docket No. DEP-5482) entitled, “An Instrument for Implanting a Sensor,” filed on Feb. 22, 2005, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference. This application of the present invention is shown solely by way of example, however. Other applications will be apparent to those skilled in the art and are considered to be within the scope of the present invention.

Both tool 24 and implants 26 and 28 contain miniature, wireless position transducers, which are described in detail hereinbelow. In this embodiment, the transducers are wireless, but the transducers may alternatively have wired connections for electrical power and communications, as shown below in FIG. 1B. Each transducer may be designed either to transmit or receive magnetic fields. The fields are used in generating position signals indicative of the transducer's location and orientation coordinates, as described hereinbelow. Tracking system 20 thus enables surgeon 22 to monitor the position of tool 24 relative to implants 26 and 28 throughout a working volume that comprises the space around and including the surgical site. Additional medical devices and tools with position transducers similar to those of implants 26 and 28 may also be used at additional locations in the area of the surgical site. For example, the use of such position transducers in a hip implant is shown in the above-mentioned U.S. patent application Ser. No. 10/029,473.

Alternatively, although the embodiment shown in the figures relates to orthopedic applications, the principles of the present invention may similarly be applied in other types of medical applications. For example, location pads 34 and 36 may be used in determining the coordinates of position transducers in invasive probes, such as catheters and endoscopes, which are inserted into the cardiovascular system and other organs of the body.

The coordinates of the transducers in tool 24 and implants 26 and 28 are determined relative to location pads 34 and 36, which are fixed to the body. The pads may conveniently be glued or strapped on to the body surface, or held against the skin by some other means. In the example shown in FIG. 1A, these pads are placed on the patient's calf and thigh, in proximity to implants 26 and 28. Alternatively, the location pads may be held away from the skin by support structures that are fixed to the body, so that the pads move with the body part to which they are in proximity.

Location pads 34 and 36 comprise magnetic field transducers, such as coils, which are used to transmit or receive magnetic fields. In other words, if the transducers in implants 26 and 28 and in tool 24 are configured to receive magnetic fields, then location pads 34 and 36 are configured as field generators. Alternatively, the location pads may be configured to receive fields generated by the position transducers in the implants and the tool. For the sake of simplicity in the description that follows, it is assumed that location pads 34 and 36 transmit the magnetic fields, which are received by the transducers in implants 26 and 28 and in tool 24. The roles of transmitter and receiver may be reversed in a straightforward manner, as will be apparent to those skilled in the art.

Surgeon 22 is generally free to place location pads 34 and 36 at any convenient location in the vicinity of the surgical site, as long as each of the pads is close enough to the implant (or implants) with which it is to communicate during the procedure so that the field strength remains sufficient to generate an acceptable signal. Typically, it is desirable for this purpose that the distance between pad and implant be kept to not more than about 20 cm, but greater or smaller distances may be appropriate depending on the size and strength of the field transducers. For example, 1 cm to 40 cm, and preferably a range from about 4 cm to about 20 cm.

The field generator coils in pads 34 and 36 generate electromagnetic fields at different, respective sets of frequencies {ω₁} and {ω₂}. Typically, the sets comprise frequencies in the approximate range of 100 Hz-30 kHz, although higher and lower frequencies may also be used. The sets of frequencies at which the coils radiate are set by a computer 38, which serves as the system controller for system 20. The frequencies {ω₁} and {ω₂} are set through frequency scanning techniques for identifying optimal frequencies respectively such as described in U.S. Pat. No. 6,373,240 which is incorporated herein by reference. For the purposes of system 20, pads 34 and 36 are placed in close proximity to the surgical site so that minimal energy is needed to generate the magnetic field. The pads are typically positioned such that the working volume of the tracking system includes the entire area in which the surgeon is operating. Furthermore, pads 34 and 36 are positioned so as not to impede access to the surgical site.

At any instant in time, the applied magnetic fields induce currents in coils contained in the transducers of tool 24 and of implants 26 and 28. The induced currents comprise components at the specific frequencies in sets {ω₁} and {ω₂}. The respective amplitudes of these currents (or alternatively, of time-varying voltages that may be measured across the transducer coils) are dependent on the location and orientation of the position transducer relative to the locations and orientations of the field generator coils. In response to the induced currents or voltages, signal processing and transmitter circuits in each position transducer generate and transmit position signals that are indicative of the location and orientation of the transducer.

These position signals are received by a wireless control unit 40, which is coupled to computer 38. Alternatively, the transducers of tool 24 and of implants 26 and 28 may be connected by wire directly to computer 38, as shown in FIG. 1B. The computer processes the received signals in order to calculate the relative location and orientation coordinates of tool 24 and of implants 26 and 28. Hereinbelow, the relative location and/or orientation of one object to another, determined in any or all of six dimensions, is referred to as the relative disposition of the two objects. Of the six dimensions, three dimensions represent the X, Y, and Z coordinates of one object relative to the other. Three additional dimensions represent the angular orientation of one object relative to the other. Disposition in one dimension, for example, may mean simply the distance between the origins of the two objects.

The disposition of the tool relative to each of the implants is calculated based on the magnetic field that is generated by the location pad on the limb in which the implant is located. In other words, in the example shown in FIG. 1A, the disposition of the tool relative to implant 26 is calculated based on the field generated by location pad 34, while the disposition of the tool relative to implant 28 is calculated based on the field generated by location pad 36. Consequently, the disposition of the tool relative to each of the implants (and hence of the bones in which the implants are located) can be determined accurately notwithstanding motion of leg 30.

Optionally, one of the location pads may also comprise a position transducer that receives the magnetic field generated by the other location pad. The signals received by this transducer may then be used by computer 38 in registering the separate, “floating” coordinate systems of the two location pads. The registration may be updated whenever leg 30 is moved. In this case, determination of the coordinates of tool 24 in the frame of reference of either of location pads 34 and 36 is sufficient to determine the disposition of the tool relative to both of implants 26 and 28.

In embodiments in which the coordinate systems of multiple location pads are mutually registered, computer 38 determines the coordinates of tool 24 using the location pad that gives the most accurate position signal. Typically, the coordinates of the tool are determined based on the magnetic field that the tool transducer receives with the least noise or interference. As the tool moves through the working volume, a magnetic field signal from a first pad may initially provide the greatest accuracy and is therefore used to determine the relative disposition of the tool and the implants. Subsequently, the field from a second pad may generate a more accurate position signal, and the tracking process is “handed-off,” such that the disposition coordinates are now determined based on the field from the second pad.

The coordinates are used by the computer in driving a display 42, which shows the dispositions of the tool, screw and other elements (such as prosthetic implants) to which position transducers have been fixed.

Whereas system 20 is shown as comprising a specific configuration of implants, tools, and body surface pads, in other embodiments of the present invention, different numbers, types and configurations of devices may used.

In other embodiments of the invention, as noted above, the generation and reception of the magnetic fields are reversed such that the coils in the implants and in the tool generate the position-responsive magnetic fields, and the body surface pads receive the fields. The relative disposition of the tool and either of the implants is determined as above, by comparing the position signals induced in pads 34 and 36 by the fields radiated from the tool and the implant. In further embodiments, any or all of the set of tools, implants, and pads may comprise transducers configured to receive and to generate magnetic fields, such that there is flexibility in selecting the coordinate system and the floating origin.

Additionally or alternatively, a field transducer 46 may be attached to a fixed frame of reference, such as an operating table 44 on which patient 32 is lying, and used as a fixed coordinate reference. Magnetic fields transmitted between fixed field transducer 46 and location pads 34, 36 on the patient's body or implants 26, 28 or both may be used to register the floating origin of the location pad coordinates or the implants respectively with the fixed frame of reference. When fixed field transducer 46 interacts with the location pads, it can comprise a transducer of the type which is used in implants 26, 28 and instrument 24. When fixed field transducer 46 interacts with implants 26, 28, the functional components of field transducer 46 by which a field is generated can be similar to those in the location pads 36, 38.

A fixed field transducer can be used to monitor movement of the patient (or of a part of the patient such as a limb). In particular, it can be used to monitor movement during preparatory steps, for example to determine the point about which a limb moves (such as the center of rotation of the femur relative to the acetabulum). It can also be used to monitor movement during a procedure. In selecting the location of a fixed field transducer, the range of movement of the patient (or of a part of the patient, such as a limb) and the required signal strength for monitoring the movement may be taken into account, to ensure that an adequately strong signal is generated when the limb moves.

When a metal or other magnetically-responsive article is brought into the vicinity of an object being tracked, such as implant 26 or tool 24, the magnetic fields in this vicinity are distorted. In the surgical environment shown in FIG. 1A, for example, there can be a substantial amount of conductive and permeable material, including basic and ancillary equipment (operating tables, carts, movable lamps, etc.), as well as invasive surgery apparatus (scalpels, scissors, etc., including tool 24 itself). The magnetic fields produced by the field generator coils may generate eddy currents in such articles, and the eddy currents then cause a parasitic magnetic field to be radiated. Such parasitic fields and other types of distortion can lead to errors in determining the position of the object being tracked.

In order to alleviate this problem, the elements of tracking system 20 and other articles used in the vicinity of the tracking system are typically made of non-metallic materials when possible, or of metallic materials with low permeability and conductivity. In addition, computer 38 may be programmed to detect and compensate for the effects of metal objects in the vicinity of the surgical site. Exemplary methods for such detection and compensation are described in U.S. Pat. Nos. 6,147,480 and 6,373,240, as well as in U.S. patent application Ser. Nos. 10/448,289, filed May 29, 2003, and 10/632,217, filed Jul. 31, 2003, all of whose disclosures are incorporated herein by reference.

FIG. 1B is a schematic, pictorial illustration showing an alternative configuration of system 20, in accordance with another embodiment of the present invention. This embodiment functions in a manner that is substantially identical to the embodiment of FIG. 1A. In FIG. 1B, however, implants 26 and 28 and location pads 34 and 36 are connected by wires to an interface unit 47. The interface unit connects the wires through to computer 38. Alternatively, the wires may connect the implants to the location pads, which then communicate (over wire or wireless connections) with control unit 40 and/or computer 38. In the embodiment shown in FIG. 1B, tool 24 still communicates with control unit 40 over a wireless connection, although the tool may alternatively have a wired connection.

Location pads 36 and 38 are marked with arrows 49, whose purpose will now be explained. In this explanation, it is assumed that the location pads transmit magnetic fields, which are sensed by the transducers in implants 26 and 28; but the considerations in the explanation are equally applicable to the reverse situation, in which the implants transmit magnetic field to the location pads. As noted above, the magnetic field transducers in the location pads and implants typically comprise coils. The transmitting coils in the location pads generate magnetic dipole fields, which induce currents in the receiving coils of the implants. The currents are indicative of the location and orientation of the receiving coils relative to the transmitting coils. Because of the symmetry of the dipole field, however, the measurement is ambiguous, i.e., the point (x,y,z) will give the same current amplitudes as (-x,-y,-z). This ambiguity may lead to errors in the determination of the location of tool 24 relative to implants 26 and 28.

In order to resolve this ambiguity, the orientation of at least one of the location pads relative to at least one of the implants is fixed in advance. For this purpose, in the present embodiment, each of the location pads is marked with an arrow 49, which is aligned in a known direction with respect to the magnetic field transducer on the location pad. Alternatively, other marks or guides may be used in aligning the location pad transducer, as will be apparent to those skilled in the art. The operator of system 20, such as surgeon 22, places pads 34 and 36 on leg 30 in such a way that arrows 49 on pads 34 and 36 point toward the locations of implants 24 and 26. (In this case, both arrows point toward the patient's knee joint.) Assuming arbitrarily that arrows 49 are aligned along the positive X-axes of the respective pads, for instance, it will then be known a priori that only coordinate readings of the implant locations with positive X-coordinate values can be correct. Thus, the orientation of the coordinate axes is known, and all ambiguity is resolved.

Although arrows 49 are marked on both location pads 34 and 36 in the embodiment of FIG. 1B, it is in fact sufficient that the orientation of one of the location pads be fixed in advance. In this case, the disambiguated coordinates of both implants may be determined initially with respect to this location pad. The orientation of the axes of the other location pad may then be determined correctly based on the known coordinates of the implants. Alternatively, other means and methods of coordinate disambiguation may be used in system 20.

FIG. 1C is a schematic, pictorial illustration showing another alternative configuration of system 20, in accordance with a further embodiment of the present invention. This embodiment functions in a manner that is substantially identical to the embodiment of FIG. 1B, except that in this case, implants 26 and 28 are connected by wires to respective location pads 34 and 36, which are connected to interface unit 47.

The wires connected to implants 26 and 28 pass through soft tissue between the bone in which the implants are located and the surface of the skin above the bone. Typically, as a joint is manipulated during an operation, there is relative lateral movement between the bone and the surface of skin, especially in larger patients. This relative movement results in an increase in the distance through the soft tissue through which the wires connected to implants 26 and 28 must pass. (The distance is smallest when the path of the wire is perpendicular to the bone surface, and increases with the angle of the wire path.) In order that the wire be able to accommodate such movement, it is desirable to flex the joint after inserting the implants into the bone in order to “pull” loose wire into the soft tissue. The loose wire is then taped down to prevent further movement.

FIG. 1C also shows a reference transducer 49, which is fixed to operating table 44. Transducer 49 receives the magnetic fields generated by location pads 36 and 38 or, alternatively, generates magnetic fields that are received by the location pads (in a manner similar to fixed field transducer 46 described above). Transducer 49 thus serves to measure the location and motion of leg 30 in the fixed frame of reference of operating table 44.

FIG. 2A is a schematic, sectional illustration showing implanted screw 48, adapted to perform the functions of the above-described implants 26 and 28, in accordance with an embodiment of the present invention. Screw 48 is implanted into a bone 50, such as the femur of patient 32. To insert the screw, surgeon 22 makes an incision through overlying soft tissue 52, and then rotates the screw into bone 50 using tool 24, for example. Alternatively, the screw may be inserted percutaneously, without prior incision. Note that in the embodiment of FIG. 2A, screw 48 has no wired connection to elements outside the body. In alternative embodiments, a wire connects screw 48 to external units, such as computer 38. The configuration and operation of the circuits in screw 48 are described hereinbelow with reference to FIGS. 3A and 3B.

FIG. 2B is a schematic, sectional illustration showing another position transducer device 54, which may similarly perform the functions of implants 26 and 28, in accordance with an alternative embodiment of the present invention. Device 54 comprises an implantable screw 56, which is coupled by wires 58 to an external unit 60. Screw 56 is inserted into bone 50 in substantially the same manner as is screw 48 (leaving wires 58 to pass out of the patient's body through soft tissue 52). External unit 60 may contain a battery or power coil to power signal processing circuitry. When screw 56 comprises coils for generating a magnetic field, external unit 60 may provide the power to these coils. External unit 60 may also provide power for a wireless transponder, or communication coil, used to transmit position signals to wireless control unit 40. Alternatively, as noted above, screw 56 may be connected by wires 58 to interface unit 47 or directly to computer 38.

FIG. 3A is a schematic, pictorial illustration of a position transducer 70 that is contained in screw 48, in accordance with an embodiment of the present invention. Alternatively, transducer 70 may be contained in or otherwise attached to other types of implants and invasive devices. Transducer 70 in this embodiment comprises three sets of coils: transducer coils 72, power coils 74, and a wireless transponder coil, or communication coil 76. Alternatively, the functions of the power and communication coils may be combined, as described in the above-mentioned U.S. patent application Ser. No. 10/029,473. Further alternatively, although communication coil 76 is shown in FIG. 3A to be wound in a plane that is perpendicular to the longitudinal axis of screw 48, the communication coil or antenna may alternatively be arranged along the length of transducer 70, roughly parallel to the longitudinal axis of the screw. Coils 72, 74 and 76 are coupled to electronic processing circuitry 78, which is mounted on a suitable substrate 80, such as a flexible printed circuit board (PCB) Details of the construction and operation of circuitry 78 are described in the above-mentioned U.S. patent application Ser. No. 10/029,473 and in the above-mentioned U.S. patent application 10/706,298.

Although for simplicity, FIG. 3A shows only a single transducer coil 72 and a single power coil 74, in practice transducer 70 typically comprises multiple coils of each type, such as three transducer coils and three power coils. The transducer coils are wound together, in mutually-orthogonal directions, on a transducer core 82, while the power coils are wound together, in mutually-orthogonal directions, on a power core 84. Alternatively, the transducer and power coils may be overlapped on the same core, as described, for example in U.S. patent application 10/754,751, filed Jan. 9, 2004, whose disclosures are incorporated herein by reference.

In another embodiment, not shown in the figures, transducer coils 72 are non-concentric. The use of non-concentric coils is described, for example, in the above-mentioned PCT Patent Publication WO 96/05768 and in the corresponding U.S. patent application Ser. No. 09/414,875. Alternatively, the position transducer may comprise only a single transducer coil or two transducer coils. Further alternatively, implants 26 and 28, and tool 24 may comprise magnetic position transducers based on sensing elements of other types known in the art, such as Hall effect transducers or magneto-resistive components.

In operation, power coils 74 serve as a power source for transducer 70. The power coils receive energy by inductive coupling from an external driving antenna (which may be a part of wireless control unit 40, shown in FIG. 1A). Typically, the driving antenna radiates an intense electromagnetic field at a relatively high radio frequency (RF), such as in the range of 13.5 MHz. The driving field causes currents to flow in coils 74, which are rectified in order to power circuitry 78. Although certain frequency ranges are cited here by way of example, those skilled in the art will appreciate that other frequency ranges may be used for the same purposes.

Meanwhile, the magnetic fields generated by location pads 34 and 36 (FIG. 1A or 1B) induce time-varying signal voltages to develop across transducer coils 72, as described above. Circuitry 78 senses the signal voltages, and generates output signals in response thereto. The output signals may be either analog or digital in form. Circuitry 78 drives communication coil 76 to transmit the output signals to a receiving antenna typically in wireless control unit 40). Additionally or alternatively, coil 76 may be used to receive control information, such as a clock signal, from a transmitting antenna (not shown) outside the patient's body.

As noted above, transducer coils 72 may alternatively be driven to generate magnetic fields, which induce position signals in the coils in the body-surface location pads. Further alternatively, position transducer 70 may be connected by wire directly to computer 38, bypassing wireless control unit 40. Typically, in the wired configuration, communication coil 74 and power coil 76 are not employed, and the respective communication and power signals are transmitted by wire.

FIG. 3B is a schematic, pictorial illustration of a position transducer 90, in accordance with another embodiment of the present invention. Transducer 90 differs from transducer 70 in that transducer 90 comprises a battery 92 as its power source, instead of power coils 74. Battery 92 may be of any suitable type, either single-use or rechargeable. In other respects, the operation of transducer 90 is substantially similar to that of transducer 70, as described above.

FIG. 4 is a schematic, pictorial illustration showing details of a body-surface location pad 94, in accordance with an embodiment of the present invention. Pad 94 is adapted to perform the functions of location pads 34 and 36 (FIG. 1A). The pad comprises a base 95, such as an adhesive patch, which permits the pad to be fixed easily and securely to the body surface in any desired location.

Body surface pad 94 comprises a magnetic field transducer 98, typically in the form of three coils wound in orthogonal directions around a cubic magnetic core. Typically, each coil comprises wire of approximately 250 microns in outer diameter, wrapped several hundred turns around the core.

Alternatively, transducer 98 may comprise a smaller or larger number of coils, and the coils may be concentric or non-concentric. Transducer 98 may be used either to transmit or to receive magnetic fields, depending on the configuration of the magnetic sensing system. Typically, when transducer 98 is configured to generate magnetic fields, the current in each coil is approximately 100 mA. Alternatively, pad 94 may comprise magnetic field transducers of other types. For example, when configured to receive magnetic fields, pad 94 may comprise magnetic position transducers based on sensing elements of other types known in the art, such as Hall effect transducers or magneto-resistive components.

Pad 94 further contains a battery 96, which powers a control circuit 97. Alternatively, battery 96 is replaced by power coils similar to power coils 74 (in FIG. 3A), which receive energy by inductive coupling from an external driving antenna (typically in wireless control unit 40). Further alternatively, pad 94 may be connected by wire to an external power source, as shown in FIG. 1B.

Typically, control circuit 97 drives transducer 98 to generate magnetic fields, as described above. Alternatively, when transducer 98 is configured to receive magnetic fields, control circuit 97 processes the signals generated by the transducer and transmits position signals to control unit 40. Pad 94 comprises a communication circuit 99 for transmitting position information to control unit 40 and/or receiving instructions from the control unit. The communication circuit may communicate with the control unit by wireless link (as described above in reference to transducer 70) or by a wire to the control unit.

In embodiments in which transducer 70 or 90 is configured to generate magnetic fields, circuitry 78 and coils 72 typically transmit a weak, narrowband signal at a precisely-controlled frequency (for example, 5 kHz). In this case, transducer 98 may comprise coils in a resonant circuit that is tuned for this frequency with very high Q. Circuit 97 may comprise a narrowband digital filter, with bandwidth as small as 20 Hz, in order to detect the weak signals that are induced in transducer 98. As a result, the receiver circuit will have very high inherent gain at the transmitted frequency, and the receiver coils used in transducer 98 may be relatively small. As a result, pad 94 itself may be made sufficiently small and flexible to be conveniently fixed to the body, as shown in FIGS. 1A and 1B. Similar principles may be used to enhance system sensitivity, and thus reduce the size of the body-surface location pad, when transducer 98 serves as the field generator and coils 72 in transducer 70 or 90 receive the fields.

Optionally, pad 94 also comprises a transducer coil 100, which may be similar in construction and operation to transducer coils 72 (FIG. 3A). When multiple location pads are applied to the body surface, as shown in the embodiments of FIGS. 1A and 1B, a second transducer coil 100 on one location pad may be used to receive magnetic fields generated by field transducer coil 98 on one or more other location pads. Alternatively, field transducer 98 itself may be used both to generate magnetic fields and to receive magnetic fields generated by other location pads. The position signals generated by transducer 98 (or by transducer 100) are processed by computer 38 to determine the relative coordinates of the two (or more) location pads in the tracking system. The separate coordinate frames of the two location pads may thus be registered with one another.

Further alternatively, when fixed reference transducer 46 is utilized by system 20 (FIGS. 1A and 1B), magnetic fields transmitted between this fixed reference transducer and the location pads may be used to register the floating origin of the location pad coordinates with the fixed frame of reference.

Optionally, the coordinate systems of multiple location pads may be “chained.” In other words, the coordinates of a first location pad may be registered with the fixed reference transducer by transmitting magnetic fields between the first location pad and the fixed reference transducer, and the coordinates of a second location pad may be registered with the first location pad by transmitting magnetic fields between the two location pads. This sort of chaining may be extended over a sequence of multiple location pads, spaced along the patient's body, to register the coordinate frames of all the pads with one another and, optionally, with a fixed reference transducer. Chaining together multiple location pads in this manner makes it possible to use smaller coils in the location pads (since there will generally be at least one coil that is relatively close to each transducer in the body). Smaller coils have smaller detection volumes, and are therefore less sensitive to field disturbance by metal objects that are not in the immediate vicinity of the transducer.

FIG. 5 is a schematic, pictorial illustration showing details of tool 24, in accordance with an embodiment of the present invention. Tool 24 comprises a handle 101 and a shaft 102. A tool transducer 104 fits snugly into a suitable receptacle inside handle 101. Transducer 104 comprises sensing and communication circuits 106, which are powered by a battery 108. Typically, circuits 106 comprise transducer coils, a communication coil and processing circuitry, as in transducer 90 (FIG. 3B). The transducer coils are similar to coils 72, generating position signals that are indicative of the location and orientation of the sensor. Alternatively, the transducer coils may be used to generate magnetic fields which induce position signals in coils in the body-surface location pads. The communication coil transmits position signals to and/or receives control information from wireless control unit 40. The operation of circuits 106 is thus similar to that of the circuits in sensors 70 and 90, although elements of circuits 106 may be made larger and consume greater power than the corresponding elements in transducers 70 and 90. In a further embodiment, circuits 106 may be connected by wire directly to computer 38, bypassing wireless control unit 40.

Tool transducer 104 may be permanently housed inside tool 24, or the transducer may alternatively be removable (to replace battery 108, for example). Because the geometry of tool 24 is known, the location and orientation of handle 101, as indicated by transducer 104, indicates precisely the location and orientation of the distal tip of shaft 102. Alternatively, the tool transducer 104 may be miniaturized and may thus be contained inside shaft 102. Optionally, the tool transducer 104 may be calibrated before use in order to enhance the precision with which the shaft position is measured.

FIG. 6 is a schematic, pictorial illustration showing body surface pads 110 and 112, each comprising multiple magnetic field transducers 114 in a ring configuration, in accordance with an alternative embodiment of the present invention. The illustration shows pads 110 and 112 mounted to leg 30 of patient 32. Each of transducers 114 typically comprises coils that transmit and/or receive magnetic fields, as described previously. Pads 110 and 112 have the form of a ring or tape, which can be attached to the patient's body near the surgical site. One or more such rings or tapes may be used for a medical procedure. In the present example, it is assumed that pads 110 and 112 are used in determining the disposition of tool 24 with respect to implants 26 and 28.

Each of transducers 114 is capable of covering only a limited portion of the working volume of the medical procedure, because of the small size and power of the transducers. The relative disposition of the tool and an implant is calculated based on the magnetic field that provides the greatest accuracy, which is typically the signal received with the least noise. As the tool moves through the working volume, a magnetic field signal from one of transducers 114 may initially provide the greatest accuracy and is therefore used to determine the relative disposition of tool and the implant. Subsequently, the field from a second transducer may become clearer (i.e., less noisy), and the tracking process is “handed-off,” such that the disposition is now determined based on the field from the second pad. Each transducer may determine the relative position of its neighbor, and the position of the transducer unit and tool may then be determined in a constant frame of reference by chaining together the sequence of transducer coordinate vectors, as described above.

Although the embodiments described hereinabove relate specifically to tracking systems that use time-varying magnetic fields, the principles of the present invention may also be applied, mutatis mutandis, in other sorts of tracking systems, such as ultrasonic tracking systems, tracking systems based on DC magnetic fields, and other tracking systems that are based on electromagnetic radiation, such as optical tracking systems. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. 

1. A tracking system, comprising: a location pad, which is configured to be attached to a body of a subject and to generate a magnetic field within the body; a first position transducer, which is adapted to be introduced into the body of the subject and responsively to the magnetic field, to generate and transmit a first position signal that is indicative of first coordinates of the first position transducer relative to the location pad; a second position transducer, which is fixed to a medical tool adapted for performing a procedure on the body of the subject and is adapted, responsively to the magnetic field, to generate and transmit a second position signal that is indicative of second coordinates of the second position transducer relative to the location pad; and a system controller, which is coupled to receive and process the first and second position signals so as to determine a disposition of the tool relative to the first position transducer.
 2. The system according to claim 1, wherein the location pad is one of at least first and second location pads, which are configured to be attached to the body at respective locations and to generate respective magnetic fields with respective ranges, and wherein the first and second position transducers are adapted respectively to generate the first and second position signals responsively to the magnetic field within whose range they are located.
 3. The system according to claim 2, wherein the system controller is adapted to determine pad coordinates of the first location pad relative to the second location pad, and thereby to register the first and second coordinates in a common reference frame.
 4. The method according to claim 3, wherein the first location pad is adapted to generate a third position signal responsively to the magnetic field of the second location pad, and wherein the system controller is coupled to receive and process the third position signal in order to determine the pad coordinates.
 5. The system according to claim 4, and comprising a third location pad, wherein the second location pad is adapted to generate a fourth position signal responsively to a magnetic field of the third location pad, and wherein the system controller is coupled to receive and process the fourth position signal in order to register the first, second and third location pads in the common reference frame.
 6. The system according to claim 2, wherein the first and second location pads are mounted on a single unit, which is configured to be fixed to the body of the subject.
 7. The system according to claim 1, and comprising a third position transducer, which is fixed in a frame of reference external to the body of the subject and is adapted to generate a third position signal responsively to the magnetic field, wherein the system controller is coupled to receive and process the third position signal so as to determine the first and second coordinates in the external frame of the reference.
 8. The system according to claim 1, wherein the location pad is adapted to be affixed to a surface of the body of the subject.
 9. The system according to claim 1, wherein the location pad comprises a plurality of concentric, orthogonal magnetic field generating coils.
 10. The system according to claim 1, wherein at least one of the first and second position transducers comprises one or more transducer coils, which are adapted to sense the magnetic fields so as to generate at least one of the first and second position signals.
 11. The system according to claim 1, and comprising a driving antenna, which is adapted to radiate a radio frequency (RF) electromagnetic field, and wherein the location pad comprises a power coil, which is coupled to receive the RF electromagnetic field and thereby to provide power for generating the magnetic field.
 12. The system according to claim 1, wherein the location pad comprises an internal power source to provide power for generating the magnetic field.
 13. The system according to claim 1, wherein the first and second position transducers comprise wireless transmitters for communicating with the system controller.
 14. The system according to claim 1, wherein the first position transducer is coupled to communicate with the system controller via a wired connection.
 15. The system according to claim 14, and comprising a first wire coupling the first position transducer to the location pad, and a second wire coupling the location pad to the system controller.
 16. A tracking system, comprising: a first position transducer, which is adapted to be introduced into a body of a subject and to generate a first magnetic field; a second position transducer, which is fixed to a medical tool adapted for performing a procedure on the body of the subject and is adapted to generate a second magnetic field; a location pad, which is configured to be attached to the body of the subject, to receive the first and second magnetic fields, and to generate and transmit first and second position signals that are indicative of respective first and second coordinates of the first and second position transducers relative to the location pad; and a system controller, which is coupled to receive and process the first and second position signals so as to determine a disposition of the tool relative to the first position transducer.
 17. The system according to claim 16, wherein the location pad is one of at least first and second location pads, which are configured to be attached to the body at respective locations, and wherein the first and second location pads are adapted to receive the first and second magnetic fields within respective ranges, so that the first and second position signals are generated by the location pad within whose range the first and second position transducers are located.
 18. The system according to claim 17, wherein the system controller is adapted to determine pad coordinates of the first location pad relative to the second location pad, and thereby to register the first and second coordinates in a common reference frame.
 19. The system according to claim 18, wherein the first location pad is adapted to generate a third magnetic field, and wherein the second location pad, responsively to the third magnetic field, generates a third position signal, and wherein the system controller is coupled to receive and process the third position signal in order to determine the pad coordinates.
 20. The system according to claim 19, and comprising a third location pad, wherein the second location pad is adapted to generate a fourth magnetic field, and wherein the third location pad, responsively to the fourth magnetic field of the third location pad, generates a fourth position signal, and wherein the system controller is coupled to receive and process the fourth position signal in order to register the first, second and third location pads in the common reference frame.
 21. The system according to claim 16, wherein the first and second location pads are mounted on a single unit, which is configured to be fixed to the body of the subject.
 22. The system according to claim 16, and comprising a third position transducer, which is fixed in a frame of reference external to the body of the subject and is adapted to generate a third magnetic field, wherein the location pad is adapted responsively to the third magnetic field to generate a third position signal, and wherein the system controller is coupled to receive and process the third position signal so as to determine the first and second coordinates in the external frame of the reference.
 23. The system according to claim 16, wherein the location pad is adapted to be affixed to a surface of the body of the subject.
 24. The system according to claim 16, wherein the location pad comprises a plurality of concentric, orthogonal coils for receiving the magnetic fields.
 25. The system according to claim 16, wherein at least one of the first and second position transducers comprises one or more transducer coils, which are adapted to generate at least one of the first and second magnetic fields.
 26. The system according to claim 16, wherein the location pad is adapted to transmit the first and second position signals over a wireless connection to the system controller.
 27. A method for position tracking, comprising: attaching a location pad to a body of a subject; introducing into the body of the subject a first position transducer; performing a procedure on the body of the subject using a medical tool, to which a second position transducer is fixed; transmitting magnetic fields between the location pad and first and second position transducers; responsively to the magnetic fields, generating first and second position signals that are indicative of coordinates of the first and second position transducers relative to the location pad; and processing the first and second position signals so as to determine a disposition of the tool relative to the first position transducer.
 28. The method according to claim 27, wherein transmitting the magnetic fields comprises transmitting the magnetic fields from the location pad, and wherein generating the first and second position signals comprises generating the first and second position signals responsively to the magnetic fields received at the first and second position transducers.
 29. The method according to claim 27, wherein transmitting the magnetic fields comprises transmitting the magnetic fields from the first and second position transducers, and wherein generating the first and second position signals comprises generating the first and second position signals responsively to the magnetic fields received at the location pad from the first and second position transducers.
 30. The method according to claim 27, wherein attaching the location pad comprises attaching first and second location pads to the body, each of the first and second location pads having a respective range, and wherein the first and second position signals are indicative of the coordinates of the first and second position transducers relative to one of the first and second location pads within whose range the first and second position transducers are located.
 31. The method according to claim 30, and comprising determining pad coordinates of the first location pad relative to the second location pad, and thereby registering the first and second coordinates in a common reference frame.
 32. The method according to claim 31, wherein determining the pad coordinates comprises generating a third position signal responsively to a magnetic field transmitted between the first and second location pads, and receiving and processing the third position signal in order to determine the pad coordinates.
 33. The method according to claim 32, wherein attaching the location pad further comprises attaching a third location pad to the body, and further comprising generating a fourth position signal responsively to a further magnetic field transmitted between the second and third location pads, and receiving and processing the fourth position signal in order to register the first, second and third location pads in the common reference frames.
 34. The method according to claim 27, wherein attaching the location pad comprises mounting the first and second location pads on a single unit, which is configured to be fixed to the body of the subject.
 35. The method according to claim 27, and comprising fixing a third position transducer in a frame of reference external to the body of the subject, generating a third position signal responsively to a magnetic field transmitted between the third position transducer and the location pad, and processing the third position signal so as to determine the first and second coordinates in the external frame of the reference.
 36. The method according to claim 35, wherein processing the third position signal comprises monitoring movement of a part of the body of the subject using the first and third position transducers.
 37. The method according to claim 27, wherein attaching the location pad comprises fixing the location pad to a surface of the body of the subject.
 38. The method according to claim 27, and comprising transmitting a radio frequency (RF) electromagnetic field to provide power for generating the magnetic fields.
 39. The method according to claim 27, wherein processing the first and second position signals comprises receiving the first and second position signals over a wireless connection. 